craig, spot on. On a standard LS1 primaries should be no bigger than 1 5/8".
Mine are 1 3/4" and I loose a heap of low down torque.But they are great size if you want to modify the engine. 1 7/8 required for blown applications.
Then there is the length 32" or 36" I think is optimum and then there is the collector.
Unfortunately Sidepipes will never be as good as a tuned under car system with a cross over...but they dont sound anywhere near as good as sidepipes.
Phil
